1. Field of the Invention
The present invention relates to the form of a discharge electrode and a power supply method employed by an RF plasma generation apparatus for use in forming semiconductor films of amorphous silicon, microcrystalline silicon, polycrystalline silicon, silicon nitride, etc. to be used in solar cells, thin-film transistors, etc. as well as for use in etching such semiconductor films.
2. Description of the Related Art
As examples of the RF plasma generation apparatus, there will be described two structures used in a plasma-enhanced chemical vapor deposition apparatus (hereinafter called xe2x80x9cPCVDxe2x80x9d or a xe2x80x9cvapor deposition apparatusxe2x80x9d) used conventionally to form thin films of amorphous silicon (hereinafter called xe2x80x9ca-Sixe2x80x9d) and thin films of silicon nitride (hereinafter called xe2x80x9cSiNxxe2x80x9d); i.e., {circle around (1)} a structure using a ladder electrode for exciting discharge; and {circle around (2)} a structure using a parallel-plate electrode for exciting discharge.
{circle around (1)} First, the structure using a ladder electrode is described. Japanese Patent Application Laid-Open (kokai) No. 236781/1992 discloses a plasma-enhanced CVD apparatus which uses a ladder-like flat coil electrode assuming any of various shapes. A typical example of this strucutre will be described with reference to FIG. 21.
As shown in FIG. 21, in this PCVD apparatus, a discharge-exciting ladder electrode (may hereinafter be called a xe2x80x9cladder electrodesxe2x80x9d) 02 and a substrate heater 03 are arranged in parallel with each other within a reaction chamber 01. An RF power of, for example, 13.56 MHz is supplied to the discharge-exciting ladder electrode 02 from an RF power source 04 via an impedance-matching unit 05.
As shown in FIG. 22 showing a perspective view of the discharge-exciting ladder electrode 02, the RF power source 04 is connected to one end of the ladder electrode 02 via the impedance-matching unit 05, whereas a grounding line 06 is connected to the other end of the ladder electrode 02, whereby the ladder electrode 02 is grounded, together with the reaction chamber 01 shown in FIG. 21.
RF power supplied to the discharge-exciting ladder electrode 02 causes generation of glow discharge plasma between the substrate heater 03 and the discharge-exciting ladder electrode 02, which are disposed within the reaction chamber 01. Then, the supplied RF power flows to the ground through the grounding line 06 of the discharge-exciting ladder electrode 02. A coaxial cable is used as the grounding line 06.
A reaction gas 08; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber 01 from unillustrated cylinders through a reaction gas feed pipe 07. The supplied reaction gas 08 is decomposed by glow discharge plasma generated by the discharge-exciting ladder electrode 02. The resulting substance is deposited on a substrate 09, which is held on the substrate heater 03 and is heated to a predetermined temperature. The gas within the reaction chamber 01 is evacuated therefrom through an evacuation pipe 010 and by means of a vacuum pump 011.
Next will be described formation of a thin film on a substrate effected by use of the above-described apparatus. As shown in FIG. 21, the vacuum pump 011 is driven so as to evacuate the reaction chamber 01. Subsequently, the reaction gas 08; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber 01 through the reaction gas feed pipe 07 so as to maintain the pressure within the reaction chamber 01 at 0.05 to 0.5 Torr.
In this state, RF power is applied to the discharge-exciting ladder electrode 02 from the RF power source 04 to thereby generate glow discharge plasma. The reaction gas 08 is decomposed by glow discharge plasma generated between the discharge-exciting ladder electrode 02 and the substrate heater 03. As a result, radicals including Si, such as SiH3 and SiH2, are generated and adhere to the surface of the substrate 09, thereby forming an a-Si thin film.
{circle around (2)} Next, the structure using a parallel-plate electrode for exciting discharge will be described with reference to FIG. 23.
As shown in FIG. 23, an RF electrode 022 and a substrate heater 023 are arranged in parallel with each other within a reaction chamber 021. An RF power of, for example, 13.56 MHz is supplied to the RF electrode 022 from an RF power source 024 via an impedance-matching unit 025. The substrate heater 023, together with the reaction chamber 021, is grounded, thereby serving as a grounding electrode. Accordingly, glow discharge plasma is generated between the RF electrode 022 and the substrate heater 023.
A reaction gas 027; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber 021 from unillustrated cylinders through a reaction gas feed pipe 026. The gas within the reaction chamber 021 is evacuated therefrom through an evacuation pipe 028 and by means of a vacuum pump 029. A substrate 030 is held on the substrate heater 023 and is heated to a predetermined temperature.
Through use of the thus-configured apparatus, a thin film is formed in the following manner. As shown in FIG. 23, the vacuum pump 029 is driven so as to evacuate the reaction chamber 021. Next, the reaction gas 027; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber 021 through the reaction gas feed pipe 026 so as to maintain the pressure within the reaction chamber 021 at 0.05 to 0.5 Torr. A voltage is applied to the RF electrode 022 from the RF power source 023 to thereby generate glow discharge plasma.
Monosilane gas contained in the reaction gas 027 supplied through the reaction gas feed pipe 026 is decomposed by glow discharge plasma generated between the RF electrode 022 and the substrate heater 023. As a result, radicals including Si, such as SiH3 and SiH2, are generated and adhere to the surface of the substrate 030, thereby forming an a-Si thin film.
However, the conventional structures {circle around (1)} and {circle around (2)} using a ladder electrode and a parallel-plate electrode, respectively, for exciting discharge involve the following problems:
{circle around (1)} An electric field generated in the vicinity of the ladder electrode 02 shown in FIG. 21 causes decomposition of the reaction gas (for example, SiH4) 08 into Si, SiH, SiH2, SiH3, H, H2, etc., thereby forming an a-Si film on the surface of the substrate 09. However, when the frequency of the RF power source is increased from current 13.56 MHz to a frequency of 30 MHz to 300 MHz (very high frequency band (hereinafter called the VHF band)) in order to increase the film deposition rate in formation of the a-Si film, uniformity of electric-field distribution in the vicinity of the ladder electrode 02 is impaired, resulting in a significant impairment in thickness distribution of the a-Si film formed on the substrate 09.
FIG. 24 shows the relationship between plasma power-source frequency and film-thickness distribution (deviation from an average film thickness) in the case of film deposition on a substrate having an area of 30 cmxc3x9730 cm effected by use of the ladder electrode 02. Uniformity (within xc2x110%) of film-thickness distribution can be reliably maintained for a substrate size, or substrate area, of about 5 cmxc3x975 cm to 20 cmxc3x9720 cm.
The structure using the ladder electrode 02 encounters difficulty in forming a uniform film through employment of the VHF band, for the following reason. As shown in Table 1, the wavelength of the VHF band ranges from 1 m to 10 m in vacuum, showing an order equivalent to that of the circuit size of the film-forming apparatus. The wavelength is shortened further in a distributed-constant line, such as the coaxial cable used to transmit power or the ladder electrode. In the case of the coaxial cable, the wavelength is shortened to 0.67 times that in vacuum. When reflections arise within the circuit due to impedance mismatch, standing waves are created such that nodes and antinodes thereof are spaced one-half line wavelength apart.
Thus, in the case of an electrode having a large area, due to voltage distribution which arises from the presence of standing waves, electric-field distribution in the vicinity of the electrode becomes nonuniform, resulting in nonuniform discharge distribution. This behavior is described in a first prior-art document (J. Appl. Phys. 54(8), 1983, p.4367). This document describes nonuniform discharge derived from one-dimensional standing-wave distribution. It is conceivable that such nonuniform discharge will arise for each electrode bar of a ladder electrode.
Since the grounding line 06 shown in FIG. 22 has a length substantially equal to a wavelength shown in Table 1, the grounding line 06 fails to yield a grounding effect, and instead serves as an open end in the case of one-fourth wavelength and as a short-circuit end in the case of one-half wavelength, thereby affecting voltage and current distributions.
Furthermore, when the VHF band is employed, voltage and current distributions which arise from stray capacitance generated between the electrode and a surrounding structure or between the electrode and a grounding plate and voltage and current distributions which arise from residual inductance along electrode bars become unignorable, resulting in worsened uniformity.
For example, an electrode bar on the order of tens of cm has an inductance of several nH, which corresponds to an impedance of several xcexa9 at 100 MHz and thus is unignorable as compared with a plasma impedance of several xcexa9.
Furthermore, when the VHF band is employed, current is less likely to flow, due to the skin effect. At 100 MHz, resistance becomes about 0.5 xcexa9 per meter, causing nonuniform discharge and current loss.
Since discharge becomes nonuniform for the reasons mentioned above, formation of a uniform film becomes difficult. Accordingly, an improvement in film deposition rate effected through an increase in the frequency of a plasma power-source is very difficult to implement in the case of a large-area substrate which is required for improvement in productivity and reduction in production cost.
Notably, since the film deposition rate in formation of an a-Si film is proportional to the square of plasma power-source frequency, studies on this subject are becoming active in scientific societies of relevant technological fields. However, no success is reported in application to manufacture of large-area substrates.
Conventionally, a source frequency of 13.56 MHz is employed in supply of power to the ladder electrode 02. In this case, connecting the core conductor of a commercially available coaxial cable to a ladder electrode bar by means of a screw raises no problem. However, in generation of plasma through employment of the VHF band, unnecessary, strong plasma is generated around the core conductor and the grounding shield of the coaxial cable.
Particularly, at high power, considerably strong plasma is generated locally. This plasma locally accelerates the film deposition rate in the vicinity of a power supply point, impairs film quality, or, in some cases, causes generation of powder.
{circle around (2)} An electric field generated between the RF electrode 022 and the substrate heater 023 shown in FIG. 23 causes decomposition of the reaction gas (for example, SiH4) into Si, SiH, SiH2, SiH3, H, H2, etc., thereby forming an a-Si film on the surface of the substrate 030. However, when the frequency of the RF power source 024 is increased from conventionally-employed 13.56 MHz to the VHF band in order to increase the film deposition rate in formation of the a-Si film, uniformity of electric-field distribution established between the RF electrode 022 and the substrate heater 023 is impaired, resulting in a significant impairment in thickness distribution of the a-Si film.
FIG. 24 is a characteristic graph showing the relationship between plasma power-source frequency and film-thickness distribution (deviation from an average film thickness) in the case of film formation on a substrate having an area of 30 cmxc3x9730 cm effected by use of the parallel-plate electrode 022. Uniformity (within xc2x110%) of film-thickness distribution can be reliably maintained for a substrate size, or substrate area, of about 5 cmxc3x975 cm to 20 cmxc3x9720 cm.
The structure using a parallel-plate electrode encounters difficulty in forming a uniform film through employment of the VHF band, for the following reason. In contrast to a ladder electrode, a parallel-plate electrode has a structure which inherently makes difficult uniform supply of a reaction gas.
This problem is described in detail in a second prior-art document (Mat. Res. Soc. Symp. Proc, Vol. 219 (1991), p. 631). When the gas 027 is supplied through the reaction gas feed pipe 026 shown in FIG. 23, the film deposition rate differs between a portion of the surface of the substrate 030 located on the near side and a portion located on the far side with respect to the reaction gas feed pipe 026.
Even in the case of the strucure, not shown, in which a reaction gas is supplied through a number of holes formed in the RF electrode 022, nonuniform film-thickness distribution tends to result, due to a small diffusion volume of the gas.
Furthermore, as in the case of the ladder electrode, when the VHF band is employed, presence of standing waves has an adverse effect on voltage distribution, resulting in nonuniform discharge. In the ladder electrode, current flow is limited to the direction of an electrode bar, and terminal impedance is determined by the grounding line. By contrast, in the parallel-plate electrode, voltage distribution and current distribution are two-dimensional, and terminal impedance changes in a complicated manner, because the entire circumference of the electrode 22 serves as an end terminal. Thus, discharge distribution becomes nonuniform and, in some cases, varies with time.
In the ladder electrode, electric-field distribution for generating plasma is inherently nonuniform around an electrode bar. Thus, even when standing waves are present to some extent, their presence is not very influential. By contrast, in the parallel-plate electrode, since plasma is generated by means of uniform electric-field distribution, a slight disturbance in uniformity of electric field results in a significant impairment in uniformity of plasma.
Accordingly, an improvement in film deposition rate effected through an increase in the frequency of a plasma power-source is very difficult to implement when a large-area substrate is employed in order to improve productivity and reduce production cost. Notably, since the film deposition rate in formation of an a-Si film is proportional to the square of plasma power-source frequency, studies on this subject are becoming active in scientific societies of relevant technological fields. However, no success is reported in application to manufacture of large-area substrates.
A third prior-art document (L. Sansonnens, et.al, Plasma Sources Sci. Technol. 6(1997), p.170) reports formation of a large-area film effected by use of a parallel-plate electrode and through employment of the VHF band. This prior-art document reports that, when an RF power of 70 MHz is supplied to the center of the parallel-plate electrode, a nonuniformity of xc2x138% results. This is equivalent to our test results shown in FIG. 24. Nonuniformity of this level is too high to enable application of this method to manufacture of solar cells and thin-film transistors.
This document also reports that, when an RF power of 70 MHz is supplied to four points of the parallel-plate electrode, uniformity is improved to xc2x118%. However, uniformity of this level is still insufficient as compared with a uniformity of xc2x110% required for manufacture of solar cells.
In addition to the above-described ladder and parallel-plate electrodes, a grid-like (lattice-like or mesh-like) discharge electrode is proposed for use in a plasma-enhanced chemical vapor deposition apparatus. FIG. 25 schematically shows the configuration of this plasma-enhanced chemical vapor deposition apparatus using a grid-like RF discharge electrode.
As shown in FIG. 25, in a PCVD 031 are disposed a material gas feed member 035 having a gas inlet 034 for introducing a material gas 033 into a vacuum chamber 032; a substrate 037 supported by a substrate support means 036, which is arranged in opposition to the material gas feed member 035 and serves as a substrate heater; and a grid-like RF discharge electrode 038 disposed between the substrate 037 and the material gas feed member 035. In FIG. 25, reference numeral 039 denotes a vacuum pump, and reference numeral 040 denotes an RF power source.
Next will be described a method for forming an amorphous thin film and a microcrystalline thin film by use of the above-mentioned vapor deposition apparatus 031. The substrate (of, for example, glass, stainless steel, or heat-resistant polymeric material) 037 is fixedly attached to the substrate support means 036 serving as a substrate heater and is then heated to a predetermined temperature (for example, 200xc2x0 C.). The vacuum chamber 032 is evacuated (to, for example, about 1xc3x9710xe2x88x926 Torr) by means of the vacuum pump 039.
Next, the material gas (for example, SiR4 gas) is introduced into the vacuum chamber 032 through the material gas inlet 034. The feed rate and the evacuation rate are adjusted so as to establish a predetermined pressure within the vacuum chamber 032 and to obtain a predetermined pressure and flow rate of the material gas (for example, 800 sccm at 0.1 Torr).
Next, RF power (for example, 800 W at 60 MHz) is supplied to the RF discharge electrode 08 from the RF power source 040, thereby generating a plasma 041 of the material gas 032 around the RF discharge electrode 038. Being activated by the plasma 041, the material gas 033 enters a radical state (for example, SiH2 and SiH3, which will be hereinafter called radicals). Radicals which have reached the surface of the substrate 037 supported by the substrate support means 036 are deposited on the surface of the substrate 037 while combining chemically, thereby forming a thin film (of, for example, amorphous silicon or microcrystalline silicon).
In recent years, solar cells of amorphous silicon, solar cells of microcrystalline silicon, and liquid crystal displays using thin-film transistors have been urged to assume a large area. Thus, an apparatus for manufacturing the same; i.e., the PCVD apparatus, shows the same tendency toward an increase in the area of a thin film to be formed.
However, an increase in the area of a thin film to be formed involves difficulty in depositing a thin film uniformly (in terms of, for example, physical properties or thickness). Major causes of this difficulty are {circle around (1)} nonuniformity of flow rate distribution of gas to be introduced onto the surface of a substrate and {circle around (2)} nonuniformity of voltage distribution on the surface of a discharge electrode.
In order to improve the film deposition rate and film quality, the power source frequency shows a tendency toward increasing. Specifically, the frequency tends to be increased from conventionally-employed 13.56 MHz to a higher frequency band (for example, 40 MHz to 200 MHz). Since wavelengths of this frequency band are of equivalent order to the size of a substrate, the presence of standing waves has an adverse effect on voltage distribution on the surface of the electrode, resulting in more marked nonuniformity of the voltage distribution and thus hindering uniform film formation over a large area.
FIG. 26 exemplifies the forms and arrangement of a material gas feed pipe 051, an RF discharge electrode 052, and a substrate 053 employed in a conventional plasma-enhanced chemical vapor deposition apparatus in order to enhance uniformity of flow-rate distribution of a supplied gas. As shown in FIG. 26, in order to feed a material gas over the entire surface of the substrate 053, the material gas feed pipe 051 is configured such that gas pipes 055, each having gas outlets 054 formed therein, are arranged in the form of a ladder and in parallel with the substrate 053.
Also, in order to generate plasma over the entire surface of the substrate 053 and not to interrupt the flow of the material gas from the gas feed pipe 051 for uniform feed of the same, the RF discharge electrode 052 is configured such that electrode bars 056 are arranged in the form of a ladder and in parallel with the substrate. As shown in FIG. 26, a power supply point 057 is located at a central portion of the RF discharge electrode 052.
As compared with an apparatus using a conventional parallel-plate electrode, the apparatus shown in FIG. 26 can improve uniformity of flow rate distribution of the supplied gas and thus has exhibited good uniformity of film thickness in depositing a large-area film at a conventional source frequency of 13.56 MHz.
However, when a source frequency falling within a frequency band higher than the conventional source frequency is used in order to form a high-quality film at higher speed, the prior-art form of an RF discharge electrode tends to involve nonuniform voltage distribution, thus encountering difficulty in generating plasma uniformly over the entirety of the RF discharge electrode.
FIG. 27 shows voltage distribution as observed when the RF discharge electrode 052 is used. As shown in FIG. 27, voltage distribution is nonuniform such that voltage corresponding to the electrode bar 056 having the power supply point 057 at a central portion thereof is relatively high, indicating that plasma is generated in a nonuniform manner.
As mentioned previously, according to the third prior-art document, in the parallel-plate PCVD apparatus, power is supplied to the center on the surface of the electrode or to four points arranged on the surface of the electrode symmetrically with respect to the center, whereby relatively uniform voltage distribution is obtained even at a source frequency of 70 MHz. However, even this prior-art apparatus exhibits a high film-thickness nonuniformity of xc2x118%, failing to obtain a sufficiently uniform thin film (within xc2x110%). Thus, there has been demand for a PCVD apparatus capable of obtaining a uniform thin film.
In view of the above-mentioned problems, an object of the present invention is to provide an RF discharge electrode capable of forming an amorphous thin film of a uniform thickness and a microcrystalline thin film of a uniform thickness, as well as to provide a plasma vapor deposition apparatus using the same.
To achieve the above object, the present invention provides an RF discharge electrode of an RF plasma generation apparatus to which power is supplied from an RF power source through a matching unit, wherein the discharge electrode has at least two RF power supply points. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby obtaining uniform film deposition rate distribution.
Preferably, two groups of electrode bars, each group comprising a plurality of parallel electrode bars, are arranged perpendicular to each other; and the RF power supply points are arranged axisymmetrically with respect to a bisector which bisects sides of the RF discharge electrode. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby obtaining uniform film deposition rate distribution.
The present invention also provides an RF plasma generation apparatus in which the discharge electrode of the present invention and substrate support means are disposed in parallel with each other within a reaction chamber, and RF power is supplied to the discharge electrode from an RF power source through a matching unit. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby enabling uniform deposition even when the area of the substrate increases.
Preferably, the power supply portion is located in a peripheral portion of the discharge electrode which does not face a substrate. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of the substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby enabling uniform deposition even when the area of the substrate increases.
Preferably, power transmission lines extending from the matching unit to the corresponding power supply points are of the same length. Thus, the same phase of RF voltage can be established at the power supply points, thereby preventing occurrence of unstable, inconsistent standing waves and enabling fine adjustment of the positions of the power supply points with relative ease in pursuit of uniform film deposition rate distribution.
Preferably, the transmission line is connected to the power supply portion of the discharge electrode such that an uninsulated bare metallic connector for connecting the power supply point and a power line of the transmission line has a diameter at least equal to that of the electrode bar as measured in the vicinity of the power supply portion. Thus, generation of intense discharge resembling corona discharge around the core conductor and around a shield can be eliminated, thereby preventing impairment in film deposition rate distribution and generation of powder.
Preferably, the distance between the power supply point and a metallic terminal member of a grounding line of the transmission line is at least 1 cm. Thus, generation of intense discharge resembling corona discharge around the core conductor and around a shield can be eliminated, thereby preventing impairment in film deposition rate distribution and generation of powder.
Preferably, the transmission line is attached to the discharge electrode from opposite the substrate at an angle of at least 45xc2x0 with respect to the discharge electrode. Thus, generation of intense discharge resembling corona discharge around the core conductor and around a shield can be eliminated, thereby preventing impairment in film deposition rate distribution and generation of powder.
Preferably, the shortest distance between a metallic portion of the power line of the transmission line and a grounded member is at least 1 cm. Thus, generation of intense discharge resembling corona discharge around the core conductor and around a shield can be eliminated, thereby preventing impairment in film deposition rate distribution and generation of powder.
Preferably, the metallic connector of the power supply portion of the discharge electrode is covered by an insulator having an outside diameter of 10 mm to 40 mm. Thus, plasma can be eliminated completely from around the metallic connector, thereby preventing impairment in uniformity of film deposition rate distribution.
Preferably, a gap between an outer circumferential surface of the metallic connector and an inner circumferential surface of the insulator is not greater than 3 mm. Thus, generation of plasma within the gap can be prevented.
Preferably, a structure for power supply to the discharge electrode is an insulated structure such that the transmission line is connected to the discharge electrode by means of a connector and a receptacle in such a manner that a radial gap between an external shield and a core conductor within the interior of the connected connector and receptacle is not greater than 3 mm. Since current is less likely to flow through an insulator in a DC electric field, discharge current is suppressed, thereby retarding occurrence of discharge.
The present invention further provides an RF plasma generation apparatus in which a discharge electrode and substrate support means are disposed in parallel with each other within a reaction chamber, and RF power is supplied to the discharge electrode from an RF power source through a matching unit. The number and position of power supply points are determined such that the shortest distance as measured along an electrode between at least one of the power supply points and any point located within a portion of a surface of the discharge electrode which faces a substrate is not greater than one-fourth of the in-vacuum wavelength of the RF power. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby obtaining uniform film deposition rate distribution.
The present invention further provides an RF plasma generation apparatus in which a discharge electrode and substrate support means are disposed in parallel with each other within a reaction chamber, and RF power is supplied to the discharge electrode from an RF power source through a matching unit. In order to supply RF power to a plurality of points on the discharge electrode, the RF plasma generation apparatus comprises, as a power transmission line, a first coaxial cable for receiving an output from the RF power source and having two divided ends; two second coaxial cables connected to the divided ends of the first coaxial cable, each of the second coaxial cables having a length equal to one-fourth of an in-cable wavelength of the RF power and a characteristic impedance equal to that of the first coaxial cable, and having two divided ends; and two third coaxial cables connected to the divided ends of each of the second coaxial cables, each of the third coaxial cables having a characteristic impedance equal to that of the first coaxial cable. Thus, a portion of power supplied from the RF generator which is lost in the course of transmission through the transmission line decreases, thereby increasing a portion of power to be used to generate plasma.
Preferably, the coaxial cable serves as a distributed constant line. Thus, a portion of power supplied from the RF power source which is lost in the course of transmission through the transmission line decreases, thereby increasing a portion of power to be used to generate plasma.
The present invention further provides a power supply method for an RF plasma generation apparatus in which a ladder-type or grid-type discharge electrode and substrate support means are disposed in parallel with each other within a reaction chamber and in which RF power is supplied to the discharge electrode from an RF power source through a matching unit. Two or more power supply points are employed for supply of power. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby obtaining uniform film deposition rate distribution.
Preferably, the number and position of the power supply points are determined such that the shortest distance as measured along an electrode between at least one of the power supply points and any point located within a portion of a surface of the discharge electrode which faces a substrate is not greater than one-fourth of the in-vacuum wavelength of the RF power. Thus, voltage distribution on the ladder electrode, which has an effect on uniformity of discharge distribution on the surface of a substrate, can be suppressed to a sufficiently low level of nonuniformity, thereby obtaining uniform film deposition rate distribution.
Preferably, in order to supply RF power to a plurality of points on the discharge electrode, as a power transmission line, a first coaxial cable having two divided ends is provided in order to receive an output from the RF power source; two second coaxial cables each having two divided ends are connected to the divided ends of the first coaxial cable, each of the second coaxial cables having a length equal to one-fourth of an in-cable wavelength of the RF power and a characteristic impedance equal to that of the first coaxial cable; and two third coaxial cables each having a characteristic impedance equal to that of the first coaxial cable are connected to the divided ends of each of the second coaxial cables. Thus, a portion of power supplied from the RF power source which is lost in the course of transmission through the transmission line decreases, thereby increasing a portion of power to be used to generate plasma.